1. Technical Field
The present invention concerns heat resistant cast steels having good thermal fatigue resistance. The heat resistant cast steel of the invention is suitable as the material for the engine parts, for example, exhaust manifolds and turbo-housings, which are used under the conditions where the part is repeatedly heated to such a high temperature as 900° C. or higher.
2. Prior Art
To date, ductile cast iron has been used as the material for the above-mentioned engine exhaust parts to which good thermal fatigue resistance is required. For the parts which are exposed to particularly high temperature exhaust gas Niresist cast iron and ferritic stainless cast steel have been used. Recently, since regulations against the exhaust gas has been getting more severe, necessitates increase in combustion efficiency of the engines, and thus, temperature of the exhaust gas is going to so high as 900° C. or higher. Therefore, austenitic stainless cast steel has been used in some fields of parts, though it has a coefficient of thermal expansion higher than that of the ferritic materials and thus, disadvantageous from the view point of thermal fatigue resistance, due to the high strength at a temperature higher than 900° C.
Known inventions concerning austenitic heat resisting cast steel are disclosed in, for example, Japanese Patent Disclosure S. 50-87916 and S. 54-58616. These steels were, however, developed for the purpose of improving high temperature strength without paying consideration on the thermal fatigue, and there has been demand for better heat resisting cast steel in regard to the thermal fatigue resistance. In order to improve the thermal fatigue resistance of the cast steel it is necessary to realize not only increase in the high temperature strength but also decrease in the coefficient of thermal expansion.
The inventors made research on Fe—Ni—Cr—W—Nb—Si—C—based cast steel and found the following relation concerning the influence of contents of the alloy components on the mean coefficient of thermal expansion the formulae of the chemical symbols contents in matrix are in weight percent, and [MC] and [M23C6] are in atomic percent):    1) Tensile strength at 1050° C.σTS at 1050° C.(MPa)=68.73−11.82Si+9.35[MC]+4.38[M23C6]    2) Mean coefficient of thermal expansion in the temperature range from room temperature to 1050° C.αRt−1050° C.×10−4(1/° C.)=21.281−0.046Ni−0.044Cr−0.135W+1.656Nb−0.192[MC]−0.082[M23C6]
It has been found that MC- and M23C6-type carbides have important influence on increase of the high temperature strength and decrease of the coefficient of thermal expansion. Further, it has been found that tungsten is used not only to contribute to the high temperature strength of the austenitic cast steel, but also to decrease in the coefficient of thermal expansion.
As the results of further research the inventors ascertained that “M” of the MC-type carbide is mainly Nb and “M” of the M23C6-type carbide is mainly Cr and W, and found that formation of MC-type carbide by Nb is useful for increase in the high temperature strength and decease in the coefficient of thermal expansion, while Nb in the matrix has negative effect. If the addition amount of MC-type carbide-forming element such as Nb is excess to C-content, formation of MC-type carbides is easier than that of M23C6-type carbides. Then, M23C6-type carbides will not be formed and the matrix contains excess Nb, which will rather result in decrease of high temperature strength and increase of thermal expansion coefficient. In the conventional austenitic heat resistant steel it has been a tendency to add excess amount of Nb, and the added Nb forms the MC-type carbide. It is the inventors' conclusion that it is advisable to have not only the MC-type carbides formed but also the M23C6-type carbides necessarily formed.
The inventors then experienced that, upon carrying out thermal fatigue tests according to JIS Z 2278 in which the samples are subjected to repeated heat cycle of 1050° C. to 150° C., significant cracks occur in cast steels having mean coefficients of thermal expansion from room temperature to 1050° C. exceeding 20.0×10−4 and tensile strength lower than 50 MPa, particularly, cast steels having 0.2%-proof stress lower than 30 MPa, and further test can no longer be continued. Thus, it is concluded that, in order to achieve sufficient thermal fatigue lives, the steel must have a mean coefficient of thermal expansion in the range from room temperature to 1050° C. not higher than 20.0×10−4 and a tensile strength in the temperature range up to 1050° C. 50 MPa or higher.